Mulcair graduated from McGill University in 1977 with degrees in common law and civil law. During his penultimate year, he was elected president of the McGill Law Students Association, and sat on the council of the McGill Student Union. He has been married to Catherine Pinhas since 1976. She is apsychologist with Turkish-Jewish heritage who was born in France, and the couple have two sons.[3]

Mulcair was President of the Office des professions du Québec (1987 to 1993), where he introduced reforms to make disciplinary hearings more transparent and successfully led a major effort to have cases of alleged sexual abuse of patients decisively dealt with.[6][7] Mulcair was also a board member of the group Conseil de la langue française, and at the time of his appointment to the Office des Professions he had been serving as President of the English speaking Catholic Council.

On November 25, 2004, Mulcair launched Quebec's Sustainable Development Plan and tabled a draft bill on sustainable development. Also included was a proposed amendment to the Quebec Charter of Human Rights and Freedoms to create a new right, the right to live in a healthy environment that respects biodiversity, in accordance with the guidelines and standards set out in the Act.[9] Mulcair's Sustainable Development Plan was based on the successful European model and was described as one of the most avant-garde in North America.[10] Mulcair followed the proposal by embarking on a 21-city public consultation tour, and the Act was unanimously adopted by the National Assembly of Quebec in April 2006.

On April 20, 2007, Mulcair confirmed that he would be running for the New Democratic Party (NDP) in the next federal election.[15][16] His presence in the front row during a speech in Montreal by NDP Leader Jack Layton in March 2007 had already led to speculations to that effect.[17] He had previously given a speech at the Federal NDP Convention in Quebec City in September 2006.

Mulcair's maternal great-grandfather was former Quebec Premier Honoré Mercier, to whom he referred when he announced his return to politics in 2007: "My great grandfather was Honoré Mercier, so what else could you expect from me." [18] Mulcair identified former Quebec Liberal Party leader Claude Ryan as his political mentor.[19]

Mulcair also became Layton's Quebec lieutenant. On June 21, 2007, in an uncontested nomination, Mulcair became the NDP's candidate in the riding ofOutremont for a by-election on September 17. Mulcair won the by-election, defeating Liberal candidate Jocelyn Coulon 48% to 29%; the seat had been a Liberal stronghold since 1935 (except for the 1988 election). Jean Lapierre suggested that Mulcair was likely aided by defecting Bloc Quebecois supporters (the Bloc candidate had finished second in the 2006 federal election). In addition, Coulon's writings had been condemned by B'nai Brith Canada, and the local Jewishcommunity in Outremont makes up 10% of the riding demographics.[20][21] The Conservatives focused their attacks on the leadership skills of Stéphane Dion, and there were allegations that Michael Ignatieff's supporters tried to sabotage the race for the Liberals to undermine Dion's leadership.[22][23][24]

He and colleague Libby Davies were jointly appointed deputy leaders of the party.[25] Mulcair was sworn in on October 12, 2007.[26]

On October 14, 2008, Mulcair was re-elected the Member of Parliament for Outremont, making him the first New Democrat to win a riding in Quebec during a federal general election. He defeated the federal Liberal candidate, Sébastien Dhavernas, by 14,348 votes to 12,005 (a margin of 6.4%).[27]

In the 2011 federal election, despite facing a strong challenge from Liberal Martin Cauchon, a former federal justice minister, Mulcair was re-elected once more with 56.4% of the popular vote, 21,916 to 9,204.

In a May 2011 Canadian Broadcasting Corporation television interview following Osama bin Laden's capture and killing in Pakistan, Mulcair was asked whether the U.S. should "release pictures of Osama bin Laden". Mulcair answered "I don't think from what I've heard that those pictures exist, and if they do I'll leave that up to the American military."[28] Asked a second time about the existence of the pictures, Muclair replied: "No, I don’t think they do [exist]. If they’ve got pictures of a cadaver, there’s probably more going on than we suspect in what happened there."[29] Mulcair's answer was initially interpreted as casting doubt on the existence of photos of bin Laden's corpse.[30][31] It received attention from American media outlets[32] and was criticized by Canadian politicians includingPaul Dewar, then NDP foreign affairs critic,[33]Chris Alexander,[30][31] and Marc Garneau.[32] In the run-up to his NDP leadership bid, Mulcair clarified that he had actually been referring to the question of whether any pictures exist showing bin Laden reaching with a gun before he was killed.[34] He added that he never doubted American forces had killed bin Laden nor that the U.S. had photographs proving bin Laden was dead.[34][35]Paul Wells of Maclean's noted that the CBC interviewer Evan Solomon failed to pose a clear question, thus leading to the miscommunication.[36]

Thomas Mulcair during a candidates debate on February 12, 2012 in Québec

Federal NDP Leader Jack Layton passed away on August 22, 2011, following a battle with cancer, and was honoured with a state funeral. Mulcair stated that Layton's death had hit him exceptionally hard, and that while he was considering a federal NDP leadership bid, he would need several weeks to make up his mind on that decision.[37]

Mulcair declared his candidacy for the federal NDP leadership at a press conference in suburban Montreal on October 13, 2011. He has attracted the support of 43 current federal MPs,[38] including Robert Chisholm[39] and Romeo Saganash,[40] the only two to have dropped out of the leadership race.